What inch 400 do I need to spine the same as a 30" 340?

Hmmmm.  Interesting question.  You're talking dynamic spine, right?  Static spine doesn't change with arrow length.

Regardless, a .400 spine equates to 78-79# static spine, whereas the 340 is about 92-94#. Assuming a commensurate difference in dynamic spine, you would have to shorten the .400 enough to achieve a 14-15# gain in dynamic spine.  Was a thread a while back in which folks provide anecdotal findings for the I spine changes in 1/4-inch increments, but I can't find it and can't remember the results. If I had to take a WAG, I'd say about an inch, but that is a WAG.  Perhaps someone who's done it will let us know.

Might also consider reducing point weight on the .400. 

 

This would not be difficult to determine on your own, and would be more accurate than someone else's experience, since the effect of a spine change is somewhat personal to the person making the change, at least in my experience.  You presumably already have a 30" .340 spine shaft.  Buy a .400 shaft and cut it to 29.75", and put the same point on it you are already using.  Shoot them side by side, and continue to cut in 1/4" increments, if necessary, until the bare shafts fly the same.  If you seem to be getting close, you might want to change to 1/8" increments for the final adjustments.

I plugged in my numbers n the newest Stu  Miller calculator using GT Traditionals.  I can shoot a .400, 29", 145 tip.
To get the same dynamic spine using the .340 spine version, I had to go to 32" shaft in order to get matching dynamic spine.